How to find the row with the maximum matching columns? - mysql

Question

If there are three tables, TableItem, TableAbcd and TablePqrs, as below
TableItem
ID item
1 item1
TableAbcd
ID Item ColA ColB ColC ColD
1 item1 A1 B1 C1 D1
TablePqrs
ID item ColA ColB ColC ColD ColValue
1 item1 A1 B1 null null 10000
2 item1 A1 B1 C1 D1 100
Here, for a given Item, There has to be just one record in the output which has the maximum columns matching in TableAbcd and TablePqrs.
Since row 1 of TableAbcd has maximum matching columns with TablePqrs row 2.
My output for join with above three tables should be,
item ColA ColB ColC ColD ColValue
item1 A1 B1 C1 D1 100
Code tried so far,
Select item, ColA, ColB, ColC, ColD, ColValue
FROM TableItem a
LEFT OUTER JOIN TableAbcd b
ON a.item = b.item
LEFT OUTER JOIN TablePqrs c
ON (b.ColA = c.ColA AND b.ColB = c.ColB AND b.ColC = c.ColC AND b.ColD = c.ColD)
OR (b.ColA = c.ColA AND b.ColB = c.ColB AND b.ColC = c.ColC)
OR (b.ColA = c.ColA AND b.ColB = c.ColB)
if fetch's me two records, i know there may be design issues, but we are getting data from third party legacy system, which has table structure as per its needs and sending this to another interface.
Please suggest.

Answer 1

Here the question is: How many columns match between B and C?
For the join clause you only need that at least one column of b matches the same column in c:
from c
left join b
on c.A = b.A or c.B = b.B or c.C = b.C or c.D = b.D
You can calc it by:
(case when c.A = b.A then 1 else 0 end)
+ (case when c.B = b.B then 1 else 0 end)
+ (case when c.C = b.B then 1 else 0 end)
+ (case when c.D = b.D then 1 else 0 end) as matches
Then simply order by matching rows (descendant) and limit the result to 1 row.
select
c.id, c.item, c.A, c.B, c.C, c.D, c.colValue,
(case when c.A = b.A then 1 else 0 end)
+ (case when c.B = b.B then 1 else 0 end)
+ (case when c.C = b.B then 1 else 0 end)
+ (case when c.D = b.D then 1 else 0 end) as matches
from c
left join b
on c.A = b.A or c.B = b.B or c.C = b.C or c.D = b.D
order by
((case when c.A = b.A then 1 else 0 end)
+ (case when c.B = b.B then 1 else 0 end)
+ (case when c.C = b.B then 1 else 0 end)
+ (case when c.D = b.D then 1 else 0 end)) desc
limit 1;
I've set up a rextester example just to check it: http://rextester.com/IPA67860

Answer 2

With TableAbcd called a and TablePqrs called p, the number of matches is (p.cola = a.cola) + (p.colb = a.colb) + (p.colc = a.colc) + (p.cold = a.cold), because in MySQL true is 1 and false is 0.
Now you are looking for the p records for which no other p record exists with a higher number of matches:
select *
from tablepqrs p1
where not exists
(
select *
from tablepqrs p2
join tableabcd a on a.item = p2.item
where p2.item = p1.item
and (p2.cola = a.cola) + (p2.colb = a.colb) + (p2.colc = a.colc) + (p2.cold = a.cold) >
(p1.cola = a.cola) + (p1.colb = a.colb) + (p1.colc = a.colc) + (p1.cold = a.cold)
);

Answer 3

In the code below you can see another option to filter. Its a similar approach to the one proposed by McNets, but using window functions.
The key is to compute a ranking which allows to determine the TablePqrs row with the best match. In the other hand, if two rows have the same ranking for the same item value, we have to use additional criteria to undo the tie. in the example, the criteria is the ID of the TableAbcd table. I'm not using outer joins so there will be no results for TableItems records without match ranking.
I'm not pretty sure if it really fits what you really want, just try it and get your own conclusions.
SELECT TableItem.id,
TableItem.item,
TablePqrs.colA,
TablePqrs.colB,
TablePqrs.colC,
TablePqrs.colD,
TablePqrs.value
FROM TableItem
INNER JOIN (SELECT DISTINCT
tableItemId,
FIRST_VALUE(tablePqrsId) OVER (PARTITION BY tableItemId ORDER BY ranking DESC, tablePqrsId DESC) tablePqrsId
FROM (SELECT rankTableItem.ID tableItemId,
rankTablePqrs.ID tablePqrsId,
CASE WHEN rankTablePqrs.colA IS NULL THEN 0 ELSE 1 END +
CASE WHEN rankTablePqrs.colB IS NULL THEN 0 ELSE 1 END +
CASE WHEN rankTablePqrs.colC IS NULL THEN 0 ELSE 1 END +
CASE WHEN rankTablePqrs.colD IS NULL THEN 0 ELSE 1 END ranking
FROM TableItem rankTableItem
INNER JOIN TableAbcd rankTableAbcd ON rankTableItem.item = rankTableAbcd.item
INNER JOIN TablePqrs rankTablePqrs ON rankTablePqrs.item = rankTableAbcd.item
AND (rankTableAbcd.colA = rankTablePqrs.colA
OR rankTableAbcd.colB = rankTablePqrs.colB
OR rankTableAbcd.colC = rankTablePqrs.colC
OR rankTableAbcd.colD = rankTablePqrs.colD))) pivotTable ON pivotTable.tableItemId = TableItem.Id
INNER JOIN TablePqrs ON TablePqrs.Id = pivotTable.tablePqrsId

Answer 4

I tried the below thing and it worked, the coalesce helps me prioritise which value to pick depending upon the order i mention in it.
Select item, ColA, ColB, ColC, ColD, ColValue
FROM TableItem a
LEFT OUTER JOIN (
SELECT item,
COALESCE(c1.ColValue,c2.ColValue,c3.ColValue) ColValue
FROM abc b
LEFT OUTER JOIN pqr c1
ON b.ColA = c1.ColA AND b.ColB = c1.ColB AND b.ColC = c1.ColC AND b.ColD = c1.ColD
LEFT OUTER JOIN pqr c2
ON b.ColA = c2.ColA AND b.ColB = c2.ColB AND b.ColC = c2.ColC
LEFT OUTER JOIN pqr c3
ON b.ColA = c3.ColA AND b.ColB = c3.ColB
GROUP BY item
) as Fact
ON Fact.item = a.item